Selective photoelectrochemical oxidation of glucose to glucaric acid by single atom Pt decorated defective TiO2

Photoelectrochemical reaction is emerging as a powerful approach for biomass conversion. However, it has been rarely explored for glucose conversion into value-added chemicals. Here we develop a photoelectrochemical approach for selective oxidation of glucose to high value-added glucaric acid by using single-atom Pt anchored on defective TiO2 nanorod arrays as photoanode. The defective structure induced by the oxygen vacancies can modulate the charge carrier dynamics and band structure, simultaneously. With optimized oxygen vacancies, the defective TiO2 photoanode shows greatly improved charge separation and significantly enhanced selectivity and yield of C6 products. By decorating single-atom Pt on the defective TiO2 photoanode, selective oxidation of glucose to glucaric acid can be achieved. In this work, defective TiO2 with single-atom Pt achieves a photocurrent density of 1.91 mA cm−2 for glucose oxidation at 0.6 V versus reversible hydrogen electrode, leading to an 84.3 % yield of glucaric acid under simulated sunlight irradiation.

× 100% (1) where J is the photocurrent density, λ is the wavelength of the incident light, and Pmono is the illumination intensity at different wavelengths. The incident photo-to-GLA conversion efficiency of the Pt/def-TiO2 photoanode at 0.6 VRHE was calculated by Supplementary Equation (2): Incident photo − to − GLA conversion efficiency = IPCE × GLA (2) where fGLA is the faradaic efficiency for GLA production.

Quantification analysis of the reaction products:
The glucose and its products from the PEC cell were analyzed by a Shimadzu LC-20AT high-performance liquid chromatography (HPLC) equipped with a refractive index detector. 5 mM H2SO4 at a flow rate of 0.6 mL min -1 was used as the mobile phase. In each analysis, ten times diluted electrolyte withdrawn from the PEC cell was injected directly into a BioRad Aminex 87H column with a column temperature of 60 ℃. The glucose and its products were identified and quantification analyzed by comparing their retention times in the chromatograms with those of the standard solution. Gaseous reduction products were analyzed using an online gas chromatograph with Ar as the carrier gas (GC, Shimadzu 2014). The conversion (XG), yield (Yproudcts), and selectivity (Sproudcts) were calculated using Supplementary Equation (3)(4)(5): where nG0, nGi, and nproducts, are the initial mole number of glucose, the residual number of glucose, and the generated mole number of the products. The μ and φ represent the stoichiometric coefficients of the reaction.
The faradaic efficiencies (fproducts) for the products were calculated by Supplementary Equation (6): where m represents the quantities of charge required for the generation of one product, F is Faraday's constant (96485.33 C mol -1 ), and Q is the total charge.   Table 1. Other acids: oxalic, acetic, and formic acids.   Supplementary Fig. 1a), respectively, based on their absorption spectra (Fig. 2e).  Fig. 4c, and Supplementary Fig. 9, all the positive slopes of the samples reveal the n-type semiconductor nature of rutile TiO2. After reduction treatment, the slope of the def-TiO2 photoanode is significantly decreased, reflecting the dramatic changes in the carrier density, which can be calculated by using
Obviously, after the reduction treatment, the electron density in the def-TiO2 has 3 orders of magnitude enhancement compared to that of TiO2. This great enhancement is due to the introduction of oxygen vacancies at the disordered shell serving as shallow donors, which can facilitate the charge transport in NRAs. 4 In addition, the flat-band potential (Efb) of the sample can be estimated by extrapolating the plots to 1/C 2 = 0.
After the reduction treatment, the carrier densities of def-TiO2, and Pt/def-TiO2 were significantly improved. As a result, the rd of def-TiO2, and Pt/def-TiO2 were limited to 3-7 nm over the test potential range, suggesting a big band bending occurring at the surface of def-TiO2, and Pt/def-TiO2. Therefore, fast extraction of holes toward the reactant at the surface/electrolyte junction could happen due to the big band bending, which helped to avoid the internal band-to-band recombination.

Supplementary Note 5.
IMPS is a convenient way of measuring the rate constants for charge transfer and recombination and mean electron transport time (τd) of an illuminated photoelectrode. [6][7][8][9] For the IMPS measurement, all the samples were illuminated with a narrow-band UV light (385 nm, 100 mW cm -2 ). As shown in Supplementary Fig. 10a, the IMPS shows a typical semicircle in the first quadrant representing the surface charge transfer and recombination process. The τt of an illuminated photoelectrode can be calculated from the frequency at the maximum of the semicircle in the fourth quadrant (fmax4). [10][11][12][13][14][15] . It follows that τd of an illuminated photoelectrode can be obtained from the optoelectrical admittance plot: τd ≈ 1/2πfmax4.  19 indicating the purity of the collected GLA. Supplementary Fig. 19, the Ti-L2,3 spectrum taken at 1 nm (O1) from the surface of the def-TiO2 nanorod shows two main L3 and L2 peaks with a separation of 5.3 eV and other three shoulder peaks, indicating that the reduction degree in the surface disorder shell is between Ti4O7 and TiO2. [20][21][22][23][24] While the Ti-L2,3 spectrum taken at 28 nm (O8) from the surface shows the typical EELS spectra of rutile TiO2. The gradual shift of the Ti-L2,3 edge towards higher energies from the surface to the inside (from O1 to O6) is related to a gradient descent of the Ti 3+ content.

Supplementary Note 8. As shown in
As a result, the def-TiO2 electrode shows a ~21 nm-thick reduction shell, and the content of oxygen vacancies is gradiently increased from the inside to the surface.
Supplementary Note 9. Supplementary Fig. 25a shows the typical 2D-HMBC NMR of glucose in an aqueous solution. 25 After 10 h, no obvious changes were observed in the 2D-HMBC NMR spectrum ( Supplementary Fig. 25b), suggesting no significant spontaneous reaction of glucose.
Supplementary Note 10. The LSV curves for GLU, GUR, and GLA oxidation over the TiO2, def-TiO2, and Pt/def-TiO2 photoanodes were tested with and without illumination in the electrolytes of 1 M KOH with 10 mM GLU, GUR, and GLA, respectively. As shown in Supplementary Fig. 32, the dark currents for the GLU, GUR, and GLA oxidations over all the photoanodes can be neglected compared to their corresponding photocurrents, suggesting that the GLU, GUR, and GLA oxidations over all the photoanodes are PEC reactions. Besides, the photocurrent densities of all the photoanodes follow this sequence: JGUR> JGLU> JGLA, further revealing the fast kinetics for GUR oxidation. The TiO2 photoanode shows much smaller differences in the JGUR, JGLU, and JGLA than the def-TiO2, and Pt/def-TiO2 photoanodes, probably due to that the cleavage of C-C bonds is the main reaction on the TiO2 photoanode. With the cleavage of C-C bonds suppressed, the def-TiO2 photoanode shows distinctly different photocurrents for the GLU, GUR, and GLA oxidations. The JGLU is obviously smaller than Jglucose (Fig. 4a) and JGUR ( Supplementary Fig. 32b), also confirming the ratelimiting step of GLU oxidation on the def-TiO2 photoanode. The further deposition of Pt SAs significantly promotes GLU oxidation ( Supplementary Fig. 32c), accelerating the conversion of GLA from GLU. Furthermore, sluggish kinetics for GLA oxidation are observed on the Pt/def-TiO2 photoanode. The accelerated GLU oxidation and sluggish GLA oxidation consequently result in a high selectivity of glucose to GLA over the Pt/def-TiO2 photoanode.